Logically Tahiti should always show the most different performance changes (for the more or less positive) realtive to Verde and Pitcairn. Look at the engine and memory configurations of Verde and Pitcairn and you'll see the ratios of Vertex engines, CU's, ROPS, and memory are very close with Pitcairn being 2x across the board (except with a little more than 2x the bandwidth). Compare Tahiti and Pitcairn however and you have the same number of Vertex Engines, 60% more CU's, the same number of ROPs and 1.8x the bandwidth. Dependant on where the bottlenecks of an application are Tahiti is always likely to show something more different than the other two ASIC's.

I assume you were answering my question about the difference between the 7970, 7870, 7770, if so thanks :) I don't pay enough attention to the mid range to low end GPU space to have drawn that conclusion easily.